function getJson(url){
    return new Promise((resolve,reject)=>{
        const xhr = XMLHttpRequest ?
        new XMLHttpRequest() : new ActiveXObject('Mscrosoft.XMLHttp')
        xhr.open('GET', url, false)
        xhr.setRequestHeader('Accept','application/json')
        xhr.onreadystatechange = function(){
            if(xhr.readyState !== 4) return
            if(xhr.status === 200 || xhr.status === 304) {
                resolve(xhr.responseText)
            } else {
                reject(new Error(xhr.responseText))
            }
        }
        xhr.send()
    })
}